The present invention relates to monitoring the operation of an optical modulator, and more particularly to methods and devices for tapping light from radiation modes of the optical modulator for monitoring the optical power in a guided mode without tapping light from the guided mode. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for tapping monitor light for controlling the bias of an optical modulator.
An integrated optical modulator is of great importance for operating a fiber optical communication system, especially for operating in the range of 2.5 to 40 Gbps (gigabits per second). An optical data signal may be generated by directly modulating an optical source, such as a semiconductor laser, through modulation of the laser""s electrical drive-current. However, high-speed direct modulation can induce wavelength fluctuation (chirp) in the optical signal, which can lead to wavelength dispersion in an optical fiber and degrade signal transmission.
Alternatively, an optical signal with significantly reduced chirp or with well controlled chirp can be generated by using a continuous-wave optical source that is externally modulated. For example, an external modulator may be a Mach-Zehnder type modulator 100, as shown in FIG. 1. Modulator 100 has an input waveguide 104, a splitting branch 108, two modulation waveguides 114 and 116, a recombination branch 120, and an output waveguide 124, all on a substrate 102. The waveguides may be formed in substrate 102 in any way, such as by selectively diffusing a metal, such as titanium (Ti), into the substrate to form a waveguide that has a higher refractive index than the surrounding material. In this case, the surrounding material acts as a lower-index medium, and light is guided to propagate along the higher-index waveguide. Alternatively, additional material layers may be deposited onto substrate 102 to act as cladding material or waveguide material.
In this example, a light source, such as a semiconductor laser diode that is not shown, provides continuous-wave light to input waveguide 104. The source light may be distributed by splitting branch 108 into two separate light fields that propagate through modulation waveguides 114 and 116 respectively, where each is individually phase modulated. The light fields are added by recombination branch 120 into output waveguide 124, and the amount of light that enters output waveguide 124 depends on the optical phase difference between the light fields from modulation waveguides 114 and 116.
More specifically, if the light fields are in phase, with zero phase difference, then all of the light may recombine into a propagating guided mode 150 that travels along output waveguide 124. Alternatively, if the light fields are out of phase, with 180 degree phase difference, then all of the light may recombine into a primary radiation mode 152. As shown in FIG. 1, primary radiation mode 152 may be anti-symmetric, with a field profile that has two lobes that are 180 degrees out of phase from each other. Primary radiation mode 152 is not guided by output waveguide 124 and may travel and diffractively spread in substrate 102. As a further alternative, if the phase difference is an intermediate value, then all of the light may be distributed between guided mode 150 and primary radiation mode 152. When the phase difference changes, the optical powers in guided mode 150 and primary radiation mode 152 vary accordingly, in a manner complementary to each other.
The phase difference depends on the optical phase of the light from each modulation waveguide 114 and 116, which in turn depends on the refractive index of each waveguide""s material. For example, the material of substrate 102 may be lithium niobate (LiNbO3) that has an electrooptic effect, and the refractive index of waveguides 114 and 116 may be electrically modulated. Modulation may be done through any type of electrode, such as a travelling-wave electrode that accommodates broadband modulation signals.
As described above, modulating the phase difference results in modulating the optical power in guided mode 150. The phase difference may be modulated about a bias point, which depends on modulation voltages applied, the wavelength of the source light, and the temperature and mechanical stress of the modulator. The bias point may drift over time, degrading the extinction ratio of modulator 100. However, the bias may be controlled through monitoring the modulator output power.
In an ideal modulator, the output power may be monitored by monitoring any portion of primary radiation mode 152. This is because light traveling through the modulator experiences no optical loss or scattering, and the optical power in guided mode 150 and that in primary radiation mode 152 are complementary. The modulation-responses of the two modes are in counter-phase, as shown in FIG. 2. Modulation quadrature points Q, of guided mode 150, and q, of primary radiation mode 152, occur at the same quadrature voltages. Furthermore, any portion of primary radiation mode 152 may be sampled, and the sample signal is complementary to the output power, with any necessary amplitude scaling applied.
However, in a practical modulator, complementarity of any one portion of primary radiation mode 152 may be destroyed. A variety of secondary radiation modes may be produced that combine with the primary radiation mode 152 to form a combined radiation field 158. Combined radiation field 158 and guided mode 150 may be different in amplitude, phase, and quadrature voltages, as shown in FIG. 3. Furthermore, the differences may vary with time. Therefore, merely monitoring part of combined radiation field 158 may not be sufficient to monitor the power of guided mode 150.
Secondary radiation modes may be caused by partial scattering of light traveling in the waveguides. Such scattering may include propagation scattering, splitting/bending scattering, and coupling scattering. Propagation scattering may be caused by the roughness of a waveguide/cladding boundary. For example, part of the light that propagates through waveguides 104, 114, 116 or 124 may be continuously scattered into substrate 102. Splitting/bending scattering occurs wherever there is a bend in a waveguide or a change in waveguide cross-section, such as at splitting branch 108 or at each bend of waveguides 114 and 116. Coupling scattering occurs when light is coupled from one waveguide into another, such as occurs in coupling from an output waveguide into an output optical fiber.
Some secondary radiation modes are modulated and some are not. For example, a secondary radiation mode generated at splitting branch 108 is not modulated, because the scattering occurs before the modulation waveguides 114 and 116. In contrast, light scattered subsequent to the modulation waveguides may be modulated. The radiation modes are not confined to the waveguides and propagate through the substrate. At the output face of modulator 100, the secondary radiation modes spatially overlap with primary radiation mode 152, and all modes add to create the combined radiation field 158. However within the combined field, only the two-lobed field of primary radiation mode 152 is complementary to guided mode 150. Optical interference among the primary and secondary radiation modes can significantly distort combined radiation field 158 and destroy its complementarity to guided mode 150.
As an example, combined radiation field 158 may be sampled by a photodetector across one lobe of the field profile and compared with guided mode 150. The power of guided mode 150 is proportional to cos2(xcfx80V/2Vxcfx80), where V and Vxcfx80 are an applied modulating signal and the half-wave voltage of the modulator respectively. The same dependence governs amplitude-modulated secondary radiation modes, such as those generated in the bends of recombination branch 120 as well as those generated at the interface between the output waveguide and fiber. In contrast, the power of primary radiation mode 152 is proportional to sin2(xcfx80V/2Vxcfx80). Thus, combined radiation field 158, sampled across one field lobe, can be expressed as:
Erad(x,y)=Epr(x,y)xc2x7sin(xcfx80V/2Vxcfx80)+Esec(x,y)xc2x7cos(xcfx80V/2Vxcfx80)xc2x7ejxcfx86xe2x80x83xe2x80x83(1) 
where the coordinate system (x,y) is defined by the photodetector surface; Epr(x,y,) and Esec(x,y) are the field distributions of the primary and secondary radiation mode, respectively; xcfx86 is the phase angle between the primary and secondary radiation fields.
The photodetector signal (photocurrent IPD) can be determined by multiplying Erad(x,y) by its complex conjugate and integrating the product over the entire photodetector surface. Therefore, IPD can be expressed as:
IPD=A[(1+xcex1)/2+{square root over ((1xe2x88x92xcex1)2/4+xcex1cos2xcfx86)}xc2x7cos(xcfx80V/Vxcfx80+xcex94"PHgr")]xe2x80x83xe2x80x83(2) 
where A is a constant of proportionality; xcex1 is the ratio of the secondary radiation mode power to that of primary radiation mode 152; and xcex94"PHgr" is given by:
xcex94"PHgr"=tanxe2x88x921(2{square root over (xcex1)}xc2x7cosxcfx86/(1xe2x88x92xcex1)).xe2x80x83xe2x80x83(3) 
Equations (2) and (3) show that combined radiation field 158 produces a photodetector signal that is shifted by xcex94"PHgr" with respect to the signal that would be caused by primary radiation mode 152 alone (in the absence of secondary radiation waves). This signal from combined radiation field 158 is not complementary to that produced by guided mode 150, as illustrated in FIG. 3. Equation (3) shows that xcex94"PHgr" is determined by the relative power xcex1 of the secondary radiation mode as well as the relative phase xcfx86 between the primary and secondary radiation modes. The relative phase "PHgr" is determined by the effective differential optical path between the two waves and, as such, is dependent on wavelength and temperature.
Under some conditions, that is when xcfx86=(2mxe2x88x921)xcfx80/2 and m is an integer, it follows from Equation (3) that cosxcfx86=0. In this case xcex94"PHgr"=0, and the presence of secondary radiation modes affects only the magnitude but not the phase of the signal from combined radiation field 158. Thus, the signal from sampled combined radiation field 158 may happen to be complementary to the power of guided mode 150 for specific pairs of wavelength and temperature values.
However, in general the two outputs are not complementary, and the signal from combined radiation field 158 cannot be used for accurate bias control. The accuracy is worst when cosxcfx86=xc2x11, that is when xcfx86=xcfx80m. In general, |cos xcfx86| is a variable that is randomly distributed between 0 and 1. Therefore, an average value of 0.5 (xcfx86=45xc2x0) may be used to evaluate the dependence of xcex94"PHgr" on the relative power xcex1, as is plotted in FIG. 4.
FIG. 4 shows that even a small fraction of secondary radiation mode power can significantly affect the phase of the combined radiation field""s modulation response. For example, even if the total of all secondary radiation mode powers is only 1% of the primary radiation mode power, then the modulation response of combined radiation field 158 can be altered in phase by as much as 8 degrees with respect to that of the guided mode 150. A phase difference of this magnitude is prohibitively high for most practical applications. Moreover, xcex1 may commonly be 1% or much more. For example, the output waveguide/fiber interface alone can create a 10% to 20% contribution to the secondary radiation mode power.
It is desirable to monitor the modulation bias point of an optical modulator 100 without tapping the guided mode 150. However, in practical modulators 100, secondary radiation waves may combine with the primary radiation mode 152 to destroy complementarity between the combined radiation field 158 and the guided mode 150. Thus, there is a need for a method or device that taps light from the combined radiation field 158 so as to form a monitor signal that is complementary to the optical power in the guided mode 150. Furthermore, although monitoring the output power of a Mach-Zehnder modulator 100 for controlling modulator bias is described above, those skilled in the art will recognize that the use of various optical modulators may benefit from monitoring power in a guided mode without tapping the guided mode signal. Accordingly, the present invention is not limited in application to Mach-Zehnder modulators or to bias control but is generally applicable to monitoring the output of modulators that distribute light between a guided output mode and a radiation mode.
The present invention provides a method and device for tapping light from the radiation modes of an optical modulator for monitoring the optical power in a guided output mode without tapping light from the guided mode. The invention allows monitoring the guided mode power, even when optical power in the radiation modes, as a function of differential phase, is not complementary to that of the guided mode. The device and method include incoherently adding photocurrents from separate portions of light from the radiation modes to form a monitor signal.
The foregoing general description and the following detailed description are merely exemplary and explanatory and are not restrictive of the invention as claimed.